Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jul 28 2024 20:18:50 FOUS30 KWBC 282018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Sun Jul 28 2024 Day 1 Valid 16Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS AND THE OHIO/TENNESSEE VALLEYS... ....16Z Update... ....Mid-South... The main adjustment to the ERO threat areas this morning was to expand the Slight Risk south to include more of northern MS and northern GA. Latest 12Z soundings, particularly out of Peachtree City, GA showed PWs topping 2.1" with 1,000 J/kg of MLCAPE present, highly saturated low-mid levels, mean cloud layer wind speeds <10 kts, and warm cloud layers deeper than 12,000ft. The region has also seen a rather wet July with Atlanta witnessing a top 10 wettest July-to-date on record. 06Z HREF probabilities featured low chances (20-30%) for rainfall totals >5" in northwest GA as well. Given the sensitive soils and sufficient moisture/instability in place, opted to extend the Slight Risk more into northwest GA this forecast cycle. ....Southeast TX... In Texas, the southeast coast of TX features 0-10cm relative soil moisture values >60% to the east of I-35 and south of I-10. Meanwhile, NAEFS shows a 500 kg/m/s IVT over south Texas (topping the 97.5 climatological percentile) that will supply the region of more than enough low-mid level moisture for diurnally driven storms to generate >2"/hr rainfall rates. Most storms will generally be scattered and pulse-like, but given the available moisture/instability and sensitive soils, a Marginal Risk for Excessive Rainfall remains in effect. ....Southern New England... In collaboration with BOX/OKX, we introduced a Marginal Risk for southern New England tonight as an anomalous 500mb low approaches from the southeast. While there is a lack of instability, GFS guidance between 00-12Z tonight shows 925mb easterly winds that are 1-1.5 STDs above normal and a ribbon of 925-850mb theta-e advection originating off the MA Capes. Modeled soundings from the 12Z HRRR and some other members of CAMs and global guidance were depicting low-mid level RH values averaging >80% between 00-12Z. The metro area along and south of I-95 has also seen some heavy rainfall over the past week or two. Given these factors, and the potential for localized flooding in urbanized communities near the Monday AM rush hour, a Marginal Risk was posted with the focus for flooding coming more tonight and into early AM Monday. Elsewhere, 12Z HREF probabilistic guidance was not as supportive in maintaining the Marginal Risk over southern NE and northern KS, but showed a swath of low chance probabilities (20-30%) for 3-hr QPF > 3-hr FFGs this afternoon in central and eastern SD as the cold front and potent 500mb shortwave trough approach from the west. Aside from minor tweaks based on new 12Z HREF probabilistic and PMM guidance, the Marginal Risk remains on track in the Southwest. Mullinax ---Previous Discussion--- ....Northern Plains... Heavy to potentially excessive rainfall is possible as a mid- and upper system ejects northeastward from the Mid-Mississippi Valley on Sunday into the western Great Lakes by early Monday morning. The inherited Slight Risk was once again expanded towards the south and east as moisture streaming northwards from the Mid-Mississippi system interacts with the strengthening of mid-level deformation. With model guidance showing 1 to 3 inches of rainfall...much of which could be delivered in an hour or two...there is the potential for flash flooding. ....Central Ohio/Tennessee Valley... Mid-level height falls approaching from the west should help focus and support showers and thunderstorms capable of producing some downpours from the afternoon into the evening...mainly across the western portion of the Ohio and Tennessee Valley. Northerly flow to the east and ridging to the east should slow the eastward progress of storms...at least initially. Overall...the GFS looks to be the most progressive while the ECMWF was on the slowest side of the guidance...leaving the NAM as a compromise in terms of position and its 1 to 2 inch amount as being reasonable given the amount of the instability and moisture. ....Southwest... The Marginal Risk in this area was extended east to once again include the Sacramento Mountains of south-central New Mexico, given the sensitivities of the soils in this area. Expect otherwise typical afternoon convection in the monsoonal moisture to develop once again, with several very sensitive areas into southern Arizona and New Mexico. Bann Day 2 Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024 ....2030Z Update... ....Ohio Valley... The primary adjustments for this forecast cycle were to scale back the Slight Risk in the Southeast to being over northern South Carolina and extend the Slight Risk farther west into the heart of the Ohio Valley. The concern over southern IN and central KY is veering low-level wind profiles and steady 925-850mb theta-e advection along the warm front may lead to back-building and training thunderstorm activity. While the placement of these storms Monday night is still unclear, a combination of HREF guidance and global guidance shows this footprint of >2" QPF within the region. 15Z RAP and 12Z ECMWF soundings over southern IN and central KY depict low-mid RH values averaging above 80% and warm cloud layers as deep as 13,000ft. 1-hr FFGs over the Ohio Valley are as low as 2" in some areas, which with the MUCAPE levels >1,000 J/kg and PWs ranging between 1-2"/hr are very much at risk of being exceeded. This training potential could extend as far east as eastern KY Monday evening and into early Monday morning.=20 Elsewhere, there was some consideration to adding a Marginal Risk=20 from the Delaware Valley on north and east into the lower Hudson=20 Valley given the increasing confidence in a swath of periodically=20 heavy rainfall. However, model soundings at the moment did not=20 contain much in the way of instability to go along with PWs that=20 were struggling to top 1.8". This area will be monitored over the=20 next 24 hours in the event a Marginal Risk is warranted. Only subtle adjustments to the Marginal Risk in the Southwest were made to account for adjustments in the QPF footprint. Mullinax ---Previous Discussion--- ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE EASTERN OHIO/TENNESSEE VALLEY TO SOUTHERN APPALACHIANS... The Slight Risk across this area was shifted=20 eastward...again...largely due to guidance shifts in the axis of=20 heaviest deterministic rainfall, which now appears to be focused=20 along much of the central and southern Appalachians, but extending=20 back across much middle and upper Ohio Valley. Given areas where=20 flash flood guidance has lowered values around 1.5 inches per hour=20 or 1.9 inches per 3 hours due to due to recent rainfall...the risk=20 of excessive rainfall persists given the amount of moisture still=20 in place. ....Upper Midwest... Rainfall should be diminishing in areal coverage and intensity as deeper moisture and the better dynamics get shunted eastward. Maintained a Marginal risk area here given some on-going rainfall early that could overlap with flash flood guidance lowered by rains on Sunday. ....Southwest US... Steering flow in the mid and upper levels begin to weaken as heights begin to rise while precipitable water values linger at or above an inch (even 1.25 inches in places over the desert). This warrants keeping the previously issued Marginal risk along/near the Arizona/New Mexico border. Bann Day 3 Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL LINGERING OVER PORTIONS OF THE EASTERN OHIO VALLEY TO THE APPALACHIANS... ....2030Z Update... The Slight Risk was reduced slightly over south-central OH where guidance had come in a little drier and was not as supportive of a Slight Risk for the moment. Also scaled back on the Marginal Risk=20 footprint over the central Appalachians given as latest guidance=20 has scaled back on the precipitation amounts for the time being=20 and flood potential will be more closely linked to how saturated=20 soils become following any thunderstorm activity on Day 2. The=20 latter consideration also applies to the northern Mid-Atlantic=20 where guidance is beginning to show better agreement on a stormy=20 day around the Delaware Valley and Poconos, but soils have been=20 trending drier in recent days and how saturated soils become=20 following Monday's potential thunderstorms will be important in=20 whether or not a Marginal Risk will be necessary in future forecast cycles. The Upper Midwest could also be in store for a potentially active afternoon and evening, but mostly progressive storm=20 motions coupled with a wide range of potential placements of the=20 heavy thunderstorm activity led to enough uncertainty in the=20 forecast to introduce a Marginal Risk. Only minor adjustments were=20 made to the Marginal Risk in the Southwest to account for the=20 latest QPF. Mullinax ---Previous Discussion--- ....Eastern US... With lingering potential for rainfall over a region of lowered Flash Flood Guidance in the eastern Ohio/Tennessee Valleys and adjacent areas of the Appalachians...opted to maintain a Slight Risk area in a similar position to that of the Slight Risk on Monday. Model QPF is actually pretty modest...but the environment remains capable of producing downpours on scales that are difficult for models to handle this far out. Surrounding the Slight Risk...kept a Marginal risk area in place for areas to the north where the ECMWF and GFS show energy in the northern stream making its way eastward across the eastern Great Lakes. The model spread on timing this energy is large enough to warrant a Marginal although an upgrade is certainly in the realm of possibilities if the large scale pattern supports more organization over a broader area...especially upstream. ....Southwest... Steering flow in the mid and upper levels and persistent monsoonal moisture will still be in place along the Arizona/New Mexico border that begins to ease northward towards southern Colorado during the period. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4-RQB8irep2ATb-Lt5XA56ozJF2x6zNrYhtn41ReZ4xV= kgCjezmncm71D4wy_njpA0-f2tAz2MJK2trE3hBBYk3hXSg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4-RQB8irep2ATb-Lt5XA56ozJF2x6zNrYhtn41ReZ4xV= kgCjezmncm71D4wy_njpA0-f2tAz2MJK2trE3hBBfZ64Sok$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4-RQB8irep2ATb-Lt5XA56ozJF2x6zNrYhtn41ReZ4xV= kgCjezmncm71D4wy_njpA0-f2tAz2MJK2trE3hBBbqHTo5k$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .